Transmission circuit



April 17, 1928.

R. V. L. HARTLEY TRANSMISSION CIRCUIT Filed Aug. 29. 1924 -90" PhaseSh/Tier Mechanical /npu7 &E T

mum/0r 04m ML/mrf/ey. by Arty Patented Apr. 17, 1928.

UNITED STATES PATENT OFFICE.

NEW YORK.

JERSEY, ASSIGNOR TO WESTERN N. Y., A CORPORATION OF :raausurssmn'cmcorr.

Application filed August 29, 1924. Serial No. 734,861.

This invention relates to wave transmission, and more particularly tothe converson of mechanical waves mto electrical "HVPH.

Heretofore it has been common to employ transmitters of the mechanicallyvariable resistance type, illustrated by the microphone, or of themechanically variable react-ance element type for converting mechanicalwaves, such as speech or sound waves, into electrical waves.Transmitters employing a mechanically variable resistance are in generalmore difiicult to control, and produce more objectionable distortionthan those employing a mechanically variable reactance element. On theother hand, the energy output from transmitters of the variablereactance type such, for example, as condenser transmitters, has beenfound to be insuflicient for many purposes.

Objects of the present invention are to overcome the above difficulties;to provide et'ticient methods of and apparatus for convert ng mechanicalwaves, for example, speech or other sound waves, into electrical waves;to provide a substitute for a microphonic transmitter; and to provide asuitable transmitter for a substation in a signaling or wavetransmission system.

In accordance with one aspect of the present invention a mechanical waveis converted into an electrical wave by mechanically controlling avariable reactance element to modulate the phase of a carrier wave inaccordance with the mechanical wave, and detecting the resultingmodulated wave by beating it with an unmodnlated carrier wave in phasequadrature with the first mentioned carrier wave. The detected wavecorresponds in character with the mechanical wave. The variablereactance for modulating the carrier wave may be a condensertransmitter, and is so associated with the carrier wave source as to notmaterially atl'ect its frequency.

In one form of the invention, an amplifier of the space current type isso connected as to generate the carrier wave and at the same timeproduce the electrical wave corresponding to the mechanical wave bydetection of the carrier wave modulated by the mechanical wave.Provision is made for decreasing the current fed from the output to theinput circuits of the amplifier and for increasing the detectingefficiency of the amplifier.

These and other novel features and objects of the invention will be moreclearly understood by reference to the following detailed description inview of the accompanying drawing, in which Fig. 1 is a schematic circuitdiagram of one form of the invention, and Fig. 2 is a modificationemploying an amplifier of the space current type.

Referring to Fig. 1, the generator 10 supplies a carrier wave to thecircuits 11 and 12. A phase shifter 13, of any suitable type, for shfting the phase of the carrier wave 90 degrees, is connected in one ofthe circuits 11 or 12, so that the carrier wave in one of these circuitsis in phase quadrature wth the wave in the other. The modulator 14connected in circuit 11 may be of any suitable type for mechanicallyacting upon circuit 11 to vary the reactive component of the impedancethrough which the carrier wave is transmitted. In the form of circuitshown a condenser transmitter 15, of well-known type, is connected inparallel with inductance 16 to form therewith a resonant circuit, tunedsubstantially to the frequency of the carrier wave from source 10.Condenser 15 variably reactively or capacitively impedes thetransmission of the carrier wave and thus varies its phase in accordancewith the mechanical wave applied thereto. Circuits 11 and 12 areconnected to the detectin device 17, of any well-known suitable typeiaving a non-linear transmission characteristic, the output circuit orline 18 being provided for transmission of the detected wave.

It is to be noted that phase modulation produces a modulated wave whichrequires a somewhat more complicated detecting process than thatrequired for detecting a modulated wave produced by amplitudemodulation. In the case of an amplitude modulated wave, that is, acarrier wave whose amplitude is varied in accordance with a modulatingwave, it is only necessary to beat together or produce a non-linearinteraction between the side-bands and an unmodulated component of theoriginal unmodnlated carrier wave. In the case of phase modulation,however, it has been found that beating of the side-bands with anunmodulated component of the original carrier wave produces nocomponents of the signal fre uency. This is apparently due to the fact tat when the phase of a carrier wave is modulated, the unmodulatedcomponent has a different relation to the modulated side bands than inthe case of amplitude modulation. For example. if the side bands areidentical in the two cases. the unmoduluted carrier component of thephase modulated carrier wave is in quadrature with the immodulatedcarrier component of the amplitude modulated carrier wave.

That is, the product of this modulation may be represented by theexpression: M== K [cos ptk sin pt cos qt] whereas the product ofamplitude modulation may be represented by the corresponding expression:M=K [sin pt+k sin pt cos qt] in each of which equations p and (1represent the carrier and modulating frequencies, respectively, whenmultiplied by the operator 2H; K sin pt and K cos pt represent theunmodulated carrier components; and K k sin pt cos qt represents theside band components. Comparing these expressions, it will be seen thatthey differ essentially only in that the phase of the unmodulatedcarrier of the first expression is displaced, relatively to its sidebands, by 90 electrical degrees, as compared with the unmodulatedcarrier component in the second expression. If the carrier and side bandcomponents of the first expression are combined in a detector whichobeys the square law, the reproduced signal waves resulting from theoperations on the two side bands will be found to cancel each other. Ifthe corresponding components of the second expression are similarlycombined, the reproduced signal waves resulting from the operations onthe two side bands will add to produce a resultant wave. A more detailedproof of the above relations may be found in applicants copendingapplication Serial No. 41,914, filed July 7, 1925, U. S. Patent No.1,633,016.

This difiiculty is overcome in the circuit of Fig. 1 by providing thecircuit 12 supplying a carrier wave in phase quadrature with theunmodulated component of the carrier wave in circuit 11.

In the operation of Fig. 1, a high frequency carrier wave from source 10is transmitted through circuit 11, the phase of the wave being modulatedin accordance with speech or sound waves, or other mechanical variationsimpressed upon condenser 15. Modulated waves from circuit 11 togetherwith the unmodulated carrier wave from circuit 12 in phasequadraturewith the wave from circuit 11 are transmitted to detector 17.The resulting detected wave corresponding in wave form to the mechanicalwave impressed on capacity 15, is transmitted to line 18. It will thusbe seen that the circuit of Fig. 1 produces an electrical wavecorrespondmg to an impressed mechanical wave, and is particular-l usefulas a telephone transmitter in a su station of a telephone or other wavetransmission system. The quality of the detected wave produced by themethod described is better than that produced in an ordinarymicroplionic transmitter by the same mechanical wave.

In Fig. 2, an electric wave amplifier 2!) of the well-known spacecurrcnttype is provided with the cathode 21. anode 22 and the grid or controlelement 23. Grid 23 together with cathode 21 constitutes a pair of inputterminals, and anode 22 together with cathode 21 constitutes a pair ofoutput terminals for the am lifier. Space current from source 24 issupplied to the amplifier through choke coil 25 which prevents thetransmission of alternating current from the output terminals of theamplifier through the source of direct current 24. Condenser 26 preventsthe passage of direct current while presenting a path of low ornegligible impedance to the passage of alternating current.

Inductance 27 of resonant circuit 27, 28 is coupled to inductance 29 toprovide a feed-back circuit coupling the output and input terminals ofthe amplifier whereby a carrier wave current'is generated, the frequencybeing determined by resonant circuit 27, 28. Inductance 30 and condensertransmitter 31 constitute a tuned circuit, preferably resonant to thefrequency of the carrier wave generated in the amplifier, the inductance30 being loosely coupled to inductance 27. Input resonant circuit 30, 31is connected in series with inductance 29 so that the voltages acrosscircuit 30, 31 and inductance 29 are in series. Owing to the loosecoupling between inductances 27 and 30, insufficient energy is passedtherebetween to support the generation of continuous oscillations.

Then carrier waves are being generated in amplifier 20, currenttransmitted through inductance 27 is approximately 90 degrees out ofphase with the voltage thereacross. The voltage induced in inductance 29is displaced substantially 90 degrees from the current in inductance 27.It will, therefore, be seen that the voltage applied to the inputterminals of the amplifier by inductance 2t issubstantially in phasewith or displaced 180 degrees from the voltage across the outputterminals of the amplifier. Inductance 29 is so poled as to support thegeneration of continuous oscillations. The voltage across resonantcircuit 30, 31 is displaced substantially 90 degrees from the voltageinduced in inductance 29. That this is true is evident when it isconsidered that, although the voltages induced in inductances 29 and 30from the inductance 27 are in the same phase, these voltages aredifferently duced in such inductance.

applied to the resonant circuit 30, 31. This circuit is in parallelresonant relation to the voltage induced in 29 and, therefore,constitutes a very large impedance for current due to this voltage; andin series resonant relation to the voltage induced in 30 and, therefore,constitutes a very much smaller impedance for the current due to thisvoltage. The voltage impressed on the grid from inductance 29,tiercfore, is substantially the same vectorily as the volta e in- Thecircu ating current produced in series resonant circuit 30, 31 by thevoltage induced in inductance 30 is in phase with such voltage, which isvery small. However, the principal voltage impressed on thegrid-filament results from the potential drop across the inductance 30and due to such circulating current. This voltage is obviously inquadrature with the circulating current and, therefore, with the voltageinduced in 30 and also with the voltage across inductance 29. Inductanceelement 29, therefore, may be looked upon as a source of carrier wavesin phase quadrature with the carrier wave transmitted to inductance 30from circuit 27, 28 which functions as a second source of carrier waves.

Mechanical vibrations, such as speech or other sound Waves, are appliedto condenser transmitter 31, thereby modulating the phase of the carrierwave transmitted from resonant circuit 30, 31. The resulting modulatedwaves are transmitted to the input terminals of the amplifier togetherwith the unmodulated carrier wave from inductance 29. It will,therefore, be seen that the waves applied to the input terminals of theamplifier are in correct phase relation to produce a detected wavecorresponding to the mechanical wave acting upon condenser 31. Thenon-linear transmission characteristic of amplifier 20 causes the wavesimpressed on the input terminals to beat together to produce a detectedwave. The detected wave is transmitted from the output terminals of theamplifier through condenser 26 and high frequency choke coils 32 to line18. Coils 32 are of low impedance to the detected wave and of highimpedance to the carrier wave.

If the circuit 30, 31 is resonant at the carrier frequency, the phasevariations will be proportional to the relative change in impedance thatwould occur if the frequency were similarly varied about a point ofsymmetry in the frequency-impedance curve which represents the characterof the impedance as well as its absolute value. That is, variationswould occur about the zero impedance point marking the mid-point of -thetransition portion of such curve between a relatively large inductivereactance and a relatively large capacitive reactance. The variation ofphase is a maximum for this resonant condition, and is also a maximumwhen the resistance of the resonant circuit is minimum. The ultimateeffect of this method of modulation, as is explained in applicantscopending application Serial No. 41,914, filed July 7, 1925, is toproduce side bands which are equivalent to the side bands produced byamplitude modulation, although the resultant unmodulated cul'l'icrcomponents are different in the two instances.

In the particular circuit disclosed there may be am litude modulation aswell as phase modu ation but, by reason of the inherently unfavorableconditions, amplitude modulation will be relatively very small. That is,amplitude modulation results from a change in absolute value ofimpedance. Under the conditions specified for this circuit, the changeof absolute value of impedance would be a minimum.

Condenser 33 prevents the transmission and consequent loss of thedetected Wave through the feed-back circuit in shunt to line 18, thecondenser being of high im pedance to the detected wave and of lowimpedance to the carrier wave. Resistance 34, or other suitableimpedance, provides a path of low impedance for the carrier wave inshunt to the output terminals of the amplifier thereby increasing thedetecting efficiency of the amplifier and at the same time reducing to aproper value the energy of the carrier current transmitted through thefeed-back circuit. Resistance 35 still further reduces the energytransmitted through the feed-back circuit.

As ordinarily employed, the mechanical wave applied to condenser 31 isof relative- 1y low frequency in comparison to the carrier wavegenerated in the amplifier, the detected wave transmitted to line 18being also of low frequency corresponding to the wave applied tocondenser 31. However, it is within the scope of the present inventionto utilize other frequency relations. While a condenser transmitter hasbeen disclosed as the modulating element for varying the phase of thecarrier wave, it is obvious that any suitable variable reactive element,or phase modulating apparatus, might be employed for this purpose.

It should be noted that exact phase quadrature between the carrier wavefrom circuit 30. 31 and the carrier wave from inductance 25). Fig. 2. orbetween the carrier waves in circuits 11 and 12, Fig. 1, is notessential to the successful operation of the system. The reproduced ordetected low frequency wave is substantially proportional to the cosineof twice the phase difference between the carrier waves, and so variesquite slowly with this difference in the neighborhood of degrees.

\Vhile the invention has been described with specific reference tocertain illustrative species, it is to be understood that the inventionmay he carried out in various ways and be embodied in various forms asbroadly defined in the appended claims.

What is claimed is:

1. The method of convertin a mechanical wave into an electrical wave wich comprises modulating the phase of a carrier wave in accordance withsaid mechanical wave, and producing the desired wave b beating togetherthe resulting modulate Wave and a carrier wave substantiall in phasequadrature with said first-mentioned carrier wave.

2. The method of convertin a mechanical wave into an electrical wavewhich comprises variabl reactively impeding the transmission 0 a carrierwave in accordance with said mechanical wave, and producing the desiredwave by beating together the resulting modulated wave and a carrier wavesubstantially in hase quadrature with said firstmentione carrier wave.

3. In a telephone transmitter, the method of operation which comprisesmodulating the phase of a carrier wave in accordance with a mechanicalwave, beating together the resultin modulated wave and a carrier wavesubstantially in phase quadrature with said first-mentioned wave, andtransmitting from said transmitter at least a portion of the resultingdetected wave.

4. In a telephone transmitter, the method of operation which comprisesvarying the phase of a carrier wave in accordance with a mechanicalwave, detecting the resulting modulated wave by beating therewith acarrier wave substantially in phase quadrature with said first-mentionedcarrier wave, and transmitting the resulting detected wave from thetransmitter.

5. The method of converting a mechanical wave into an electrical, wavewhich comprises separately transmitting two carrier wave currentssubstantially in phase quadrature with each other, modulatin the phaseof one of said currents in accor ance with said mechanical wave, andbeating together the resulting currents from the two phases to obtainthe desired Wave.

6. In apparatus for converting mechanical waves into electrical waves,the method of operation which comprises producing a high frequencycarrier wave current, varying the phase of said wave in accordance withthe instantaneous amplitudes of said mechanical waves, beating togetherthe resultin modulated wave and an unmodulated carrier wavesubstantially in phase quadrature with said first-mentioned carrierwave, and transmitting the resulting detected wave from the apparatus.

7. The method of utilizing a space current device of the kind having ananode, a. cathode and a control element, which comprises generating acarrier wave through the agency of said device, deriving from saidcarrier wave a second carrier wave substantially in hase quadrature withsaid first wave, mo ulating the phase of one of said carrier waves inaccordance with a mechanical wave, beating the resulting modulated wavewith said other carrier wave, and utilizing the resulting detected wave.

8. 'In a substation for an electrical wave transmission system, a air ofcircuit elements, means for producing in said elements waves which arealike except relatively displaced in phase substantially 9O electricaldegrees, one of said elements includin means whereby it ma bemechanically act upon to modulate t c phase of the wave therein inaccordance with the wave to be transmitted from the substation, andmeans for beating the resulting modulated wave with the wave produced inthe other element.

9. In a substation for an electrical wave transmission system, a pair ofcircuit elements, means for producing in said elements electrical waveswhich are alike except relatively displaced in phase substantially 90electrical degrees one of said elements including means whereby it maybe mechanically acted upon to modulate the phase of the wave therein inaccordance with the wave to be transmitted from the substation, meansfor beating the resultin modulated wave with the wave produce in theother element, and a air of output terminals for transmitting rom saidsubstation the resulting detected wave.

10. In a substation for an electrical wave transmission 5 stem, a pairof circuit elements, means or producing in said elements electricalwaves which are alike except relatively displaced in phase substantially90 electrical degrees, one of said elements including means whereby itmay be mechanically acted upon to modulate the phase of the wave thereinin accordance with the wave to be transmitted from the substation, and aspace current device connected to said circuits for beating theresulting modulated wave with the wave produced in the other element.

11. In a substation for an electrical wave transmission system, a air ofcircuit elements, means for producing in said elements electrical wavesalike except relatively displaced in phase substantially 90 electricaldegrees, one of said elements including a condenser transmitter formodulating the phase of the wave therein in accordance with the wave tobe transmitted from the substation, and means for beating the resultingmodulated wave with the wave produced in the other element.

12. In apparatus for converting a mechanical wave into an electricalwave, an electrical wave amplifier having input ter minals and outputterminals, means coupling said output and input terminals whereby acarrier wave is generated by said amplifier, said coupling meansincluding means for modulating sa d carrier wave in accordance with saidmechanical Wave, and for transmitting the resulting modulated wave tosaid amplifier for detection to produce the desired electrical wave, andmeans associated with said coupling means for decreasing the energytransmitted therethrough and for increasing the detecting efficiency ofthe amplifier.

13. In apparatus for converting a mechanical wave into an electricalwave, an electric wave ampl lier having input terminals and outputterminals. means coupling said output and input terminals whereby acarrier wave is generated by said amplifier, said coupling meansincluding means for modulating said carrier wave in accordance with saidmechanical wave, and for transmitting the resulting modulated wave tosaid amplifier for detection to produce the desired electrical wave, andmeans for transmitting the detected wave separately from said carrierwave.

14.111 apparatus for converting a mechanical Wave into an electricalwave, an electrical wave amplifier having input terniinals and outputterminals, means coupling said output and input terminals whereby acarrier wave is generated by said amplifier, said coupling meansincluding means for modulating said carrier wave in accordance with saidmechanical wave, and for transmitting the resulting modulated wave tosaid amplifier for detection to produce the desired electrical wave, anda resistance path in shunt to said coupling means for decreasing theenergy transmitted therethrough and for increasing the detectingefliciency of the amplifier.

15. In apparatus for-converting a mechanical wave into an electricalwave, an electrical wave amplifier having input terminals and outputterminals, means coupling said output and input terminals whereby acarrier wave is generated by said amplifier, said coupling meansincluding means for modulating said carrier wave in accordance with saidmechanical wave, and for transmitting the resulting modulated wave tosaid amplifier for detection to produce the desired electrical wave, anda path of relatively low impedance to said carrier wave in shunt to saidcoupling means for decreasing the energy transmitted therethrough andfor increasing the detecting etficiency of the amplifier.

16. In apparatus for converting a mechanical wave into an electricalwave, an electrical wave amplifier having input terminals and outputterminals, means coupling said output and input terminals whereby acarrier wave is generated by said amplifier, said coupling meansincluding means for modulating said carrier wave in accordance with saidmechan cal wave, and for transmitting the resulting modulated wave tosaid amplifier for detection to produce the desired electrical wave, anda path of relatively low impedance to said carrier wave and of highimpedance to the detected wave in shunt to said coupling means fordecreasing the energy transmitted therethrough and for increasing thedetecting effic ency of the amplifier.

17. In apparatus for converting a mechanical wave into an electricalwave, an electrical wave amplifier having input terminals and outputterminals. means coupling said output and input terminals whereby acarrier wave is generated by said amplifier, said coupling meansincluding means for i'nodulating said carrier wave in accordance withsaid mechanical wave. and for trans mitting the resulting modulated waveto said amplifier for detection to produce the desired electrical wave,and means associated with said coupling means for decreasing the energytransmitted therethrough and for increasing the detecting efiicicncy ofthe amplifier, said means including a resistance path in shunt to saidcoupling and a resistance path in series therewith.

18. In apparatus for converting a mechanical wave into an electricalwave, an electrical wave amplifier having input and output terminals,means coupling said output and input terminals whereby two carrier wavessubstantially in phase quadrature with each other are generated by saidamplifier and transmitted to said input terminals, said coupling meansincluding means for modulating the base of one of said carrier waves inaccor ance with said mechanical wave, and for transmitting the resultingmodulated wave to said amplifier for de tection to produce the desiredelectrical wave.

19. In apparatus for converting mechanical waves into electrical waves.an electrical wave generator. a circuit associated with said generatorfor deriving therefrom, an auxiliary wave substantially in phasequadrature with the generated wave, said circuit including means forvarying the phase of the Wave transmitted therefrom in accordance withsaid mechanical waves, said circuit being connected to apply theresulting modulated wave to said generator whereby detected wavescorresponding to said mechanical waves are produced in the generator.

20. In apparatus for converting a mechanical wave into an electricalwave, an amplifier having an anode, a cathode and a control element. afrequency determining feed-back circuit coupling said anode and controlelement whereby a carrier wave is generated by said ampli er, meansassociated with said feed-back circuit for deriving from the generatedwave a wave substantially in till phase quadrature therewith, said meansincluding a modulator for varying the phase of the wave transmittedtherethrough, and being arranged to apply the resulting modulated waveto said control element whereby a detected wave corresponding to saidmechanical wave is produced in said amplifier.

21. In apparatus for converting a mechanical wave into an electricalwave, an amplifier having a pair of in ut terminals ant a pair of outputtermina s, a resonant frequency determining circuit connected to saidoutput terminals, an inductance connectcd to said input terminals andcoupled to said resonant circuit whereby a carrier wave is generated bysaid amplifier, and a circuit substantially resonant to said carrierwave connected to said input terminals and loosely coupled to saidoutput resonant circuit, said input resonant circuit including avariable reactance controlled by said me- I 1,eee,'rae

chanical wave for modulating a carrier wave transmitted therethrough,whereby a modulated wave is transmitted to said amplifier to produce thedesired wave by detection.

22. In apparatus for converting a mechanical wave into an electricalwave, a source of two carrier waves substantially in phase quadraturewith each other, a pair of circuits connected to said source, one foreach of said waves, means for mechanically acting upon one of saidcircuits to vary in accordance with said mechanical wave the phase ofthe carrier wave transmitted therethrough, and means connected to saidcircuits for beating together the resulting waves thereby to produce thedesired wave.

In witness whereof, I hereunto subscribe 1135411111116 this 26th day ofAugust A. D

RALPH V. L. HARTLEY.

lfatent No. 1,666,138.

RALPH V. L. HARTLEY.

Granted April 11, 1928, to

- ltle hereby certified that 'error appears in the above numbered patentrequiring correction as follows:

In the drawings Fig. 2, the diagonal line extending through the numeral22, under the numeral 25, and terminating near the numeral 18, shouldnot appear on the drawing; and that the said Letters Patent should' beread with this correction therein that the same may conform to therecord of the case in the Patent Off ice.

' Signed and scaled-this 25th day of September, A. D. 1928.

M. J. Moore, Acting Commissioner of Patents.

phase quadrature therewith, said means including a modulator for varyingthe phase of the wave transmitted therethrough, and being arranged toapply the resulting modulated wave to said control element whereby adetected wave corresponding to said mechanical wave is produced in saidamplifier.

21. In apparatus for converting a mechanical wave into an electricalwave, an amplifier having a pair of in ut terminals ant a pair of outputtermina s, a resonant frequency determining circuit connected to saidoutput terminals, an inductance connectcd to said input terminals andcoupled to said resonant circuit whereby a carrier wave is generated bysaid amplifier, and a circuit substantially resonant to said carrierwave connected to said input terminals and loosely coupled to saidoutput resonant circuit, said input resonant circuit including avariable reactance controlled by said me- I 1,eee,'rae

chanical wave for modulating a carrier wave transmitted therethrough,whereby a modulated wave is transmitted to said amplifier to produce thedesired wave by detection.

22. In apparatus for converting a mechanical wave into an electricalwave, a source of two carrier waves substantially in phase quadraturewith each other, a pair of circuits connected to said source, one foreach of said waves, means for mechanically acting upon one of saidcircuits to vary in accordance with said mechanical wave the phase ofthe carrier wave transmitted therethrough, and means connected to saidcircuits for beating together the resulting waves thereby to produce thedesired wave.

In witness whereof, I hereunto subscribe 1135411111116 this 26th day ofAugust A. D

RALPH V. L. HARTLEY.

lfatent No. 1,666,138.

RALPH V. L. HARTLEY.

Granted April 11, 1928, to

- ltle hereby certified that 'error appears in the above numbered patentrequiring correction as follows:

In the drawings Fig. 2, the diagonal line extending through the numeral22, under the numeral 25, and terminating near the numeral 18, shouldnot appear on the drawing; and that the said Letters Patent should' beread with this correction therein that the same may conform to therecord of the case in the Patent Off ice.

' Signed and scaled-this 25th day of September, A. D. 1928.

M. J. Moore, Acting Commissioner of Patents.

